[Nsi-wg] multi-point proposal

[Henrik Thostrup Jensen]

Hi

On Fri, 2 Jun 2017, Guy Roberts wrote:

> Hello NSI team,
> 
> There was a discussion at Monday’s meeting about revisiting the idea of an NSI multi-point service.
> 
> I have attached the presentation by Miyamoto where he presented a proposal for such a service in our Chicago meeting.

While the base idea is fairly simple (which is good) and mixes well into 
NSI, there is a pressing real-world issue:

- What happens when someone create multiple switching points and hooks
   them together?

- In particular, when/if a loop is created? (and this is typically needed
   for redundancy in the real world - especially since current NSI is
   based off static demarcation points which make link failover rather
   tricky to engineer). OpenFlow is not really enough here.

AFAIK the only cross-platform protocol that solve this is spanning tree, 
but it is rather limited with how it works across VLANs which is suggested 
- typically an STP process is run for every VLAN. You can do VLAN rewrite, 
but I think that would cause more issues than it solves here.

Spanning tree typically becomes a pain in large environments, and mixing 
implementations across multiple vendors over high-latency links is very 
likely to cause a lot of issues (this is typically avoided).

VPLS is a lot better (as it runs over MPLS/IP, but therefore the equipment 
is a lot more expensive). Then there is stuff like TRILL, which might or 
might not be better, but it is not that widespread yet AFAIK.

However none of these suggestions mix that well with OpenFlow, which is 
the suggested switching technology in the presentation. But not having any 
established protocol for handling loops sounds a bit dangerous. It is 
infinitely easier cause accidents with multi-point than point-point.

Of course there is nothing that prevents someone from doing it - and 
something very similar has already been done already in GTS. But it also a 
more isolated infrastructure, and intended for experiments (from the 
presentation it is not quite clear what the scope of the suggestion is).


     Best regards, Henrik

  Henrik Thostrup Jensen <htj at nordu.net>
  Software Developer, NORDUnet